Negative electrode for a rechargeable lithium battery and rechargeable lithium battery including the same

ABSTRACT

A negative electrode and a rechargeable lithium battery, the negative electrode including a current collector; and a negative active material layer on the current collector, the negative active material including a carbon negative active material; wherein: an electrode density of the negative electrode is in the range of about 1.0 g/cc to about 1.5 g/cc, and a DD (Degree of Divergence) value as defined by the following Equation 1 is about 24 or greater,DD(Degree of Divergence)=(Ia/Itotal)*100  [Equation 1]wherein, in Equation 1, Ia is a sum of peak intensities at non-planar angles measured by XRD using a CuKα ray, and Itotal is a sum of peak intensities at all angles measured by XRD using a CuKα ray.

SUMMARY

The embodiments may be realized by providing a negative electrode for a rechargeable lithium battery, the negative electrode including a current collector; and a negative active material layer on the current collector, the negative active material including a carbon negative active material; wherein: an electrode density of the negative electrode is in the range of about 1.0 g/cc to about 1.5 g/cc, and a DD (Degree of Divergence) value as defined by the following Equation 1 is about 24 or greater,

DD(Degree of Divergence)=(I _(a) /I _(total))*100  [Equation 1]

-   -   wherein, in Equation 1, I_(a) is a sum of peak intensities at         non-planar angles measured by XRD using a CuKα ray, and         I_(total) is a sum of peak intensities at all angles measured by         XRD using a CuKα ray.

The DD value of the negative electrode may be about 24 to about 70.

The DD value of the negative electrode may be about 24 to about 60.

The electrode density of the negative electrode may be about 1.0 g/cc to about 1.4 g/cc.

The electrode density of the negative electrode may be about 1.0 g/cc to about 1.2 g/cc.

The negative electrode may have a single surface loading level (L/L) of about 6 mg/cm² to about 65 mg/cm².

The negative active material layer may have a thickness of a single surface except a substrate of about 40 μm to about 650 μm.

The negative active material layer may have a thickness of a single surface except a substrate of about 40 μm to about 450 μm.

The negative active material layer may have a thickness of a single surface except a substrate of about 40 μm to about 200 μm.

The I_(a) may be a sum of peak intensities at 2θ=42.4±0.2°, 43.4±0.2° 44.6±0.2°, and 77.5±0.2° measured by XRD using a CuKα ray, and the I_(total) may be a sum of peak intensities at 2θ=26.5±0.2°, 42.4±0.2°, 43.4±0.2°, 44.6±0.2°, 54.7±0.2° and 77.5±0.2° measured by XRD using a CuKα ray.

The peak intensities may be peak integral area values.

The carbon negative active material may be artificial graphite or a mixture of artificial graphite and natural graphite.

The negative active material layer may further include a Si negative active material, a Sn negative active material, a lithium vanadium oxide, or a combination thereof.

The negative electrode may have an active region facing a positive electrode and an inactive region not facing the positive electrode, and a DD value of the inactive region may be about 24 or greater.

The embodiments may be realized by providing a rechargeable lithium battery including the negative electrode according to an embodiment; a positive electrode; and an electrolyte.

The rechargeable lithium battery is a high power battery.

BRIEF DESCRIPTION OF THE DRAWINGS

Features will be apparent to those of skill in the art by describing in detail exemplary embodiments with reference to the attached drawings in which:

FIG. 1 illustrates a schematic view showing orientations of negative active materials according to an embodiment.

FIG. 2 illustrates a view showing active and inactive regions of a negative electrode of a rechargeable lithium battery according to an embodiment.

FIG. 3 illustrates a schematic view showing the structure of a rechargeable lithium battery according to an embodiment.

FIG. 4 illustrates a graph showing XRD peaks of the negative electrode according to Example 1 measured using a CuKα ray.

FIG. 5 illustrates a graph showing XRD peaks of the negative electrode according to Comparative Example 1 measured using a CuKα ray.

FIG. 6 illustrates a graph showing capacity retentions obtained by charging and discharging the rechargeable lithium battery cells manufactured according to Examples 1 to 4 and Comparative Examples 1 to 4 while changing C-rates.

FIG. 7 illustrates a graph showing cycle-life characteristics of a rechargeable lithium battery cells manufactured according to Examples 1 to 4 and Comparative Examples 1 to 4.

DETAILED DESCRIPTION

Example embodiments will now be described more fully hereinafter with reference to the accompanying drawings; however, they may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey exemplary implementations to those skilled in the art.

In the drawing figures, the dimensions of layers and regions may be exaggerated for clarity of illustration. It will also be understood that when a layer or element is referred to as being “on” another layer or element, it can be directly on the other layer or element, or intervening layers may also be present. Further, it will be understood that when an element is referred to as being “under” another element, it can be directly under, and one or more intervening elements may also be present. In addition, it will also be understood that when an element is referred to as being “between” two element, it can be the only layer between the two elements, or one or more intervening elements may also be present. Like reference numerals refer to like elements throughout.

A negative electrode for a rechargeable lithium battery according to an embodiment may include a current collector; and a negative active material layer on the current collector and including a carbon negative active material. A DD (Degree of Divergence) value defined by Equation 1, below, may be, e.g., about 24 or greater and an electrode density may be, e.g., in the range of about 1.0 g/cc to about 1.5 g/cc.

DD(Degree of Divergence)=(I _(a) /I _(total))*100  [Equation 1]

In Equation 1,

I_(a) is a sum of peak intensities at non-planar angles measured by XRD using a CuKα ray, and

I_(total) is a sum of peak intensities at all angles measured by XRD using a CuKα ray.

Herein, the non-planar angles denote 2θ=42.4±0.2°, 43.4±0.2°, 44.6±0.2°, and 77.5±0.2° when measured by XRD using a CuKα ray, that is, a (100) plane, a (101)R plane, a (101)H plane, and a (110) plane. In general, graphite has a structure classified into a hexagonal structure and a rhombohedral structure having an ABAB type stacking sequence of stacking graphene layers, and the R plane denotes the rhombohedral structure, while the H plane denotes the hexagonal structure.

In addition, all the angles denote 2θ=26.5±0.2°, 42.4±0.2°, 43.4±0.2° 44.6±0.2°, 54.7±0.2°, and 77.5±0.2° when measured by XRD using a CuKα ray, that is, a (002) plane, a (100) plane, a (101)R plane, a (101)H plane, a (004) plane, and a (110) plane. A peak at 2θ=43.4±0.2° may appear when a peak of a (101)R plane of a carbon material is overlapped with another peak of a (111) plane of a current collector, for example, Cu.

In general, peak intensity indicates a height of a peak or an integral area of the peak, and according to an embodiment, the peak intensity indicates the integral area of a peak.

In an implementation, the XRD is measured under a measurement condition of 2θ=10⁰ to 80°, a scan rate (°/S) of 0.044 to 0.089, and a step size (°/step) of 0.013 to 0.039 by using a CuKα ray as a target ray but removing a monochromator to improve a peak intensity resolution.

In an implementation, the DD value of the negative electrode may be about 24 or greater, e.g., about 24 to about 70 or about 24 to about 60.

The DD value within the above ranges means that the negative electrode active material is not sufficiently laid horizontally with the current collector but is sufficiently oriented to facilitate movement of Li ions in the negative electrode, e.g., control of random orientation. When the DD value is less than 24, the DC internal resistance may be increased and rate capability, particularly high rate capability and cycle-life characteristics, may be deteriorated.

Further, within the ranges of the DD value, it is possible to help suppress an increase in resistance at the discharge end, to minimize a DC internal resistance (DC-IR), and to exhibit improved cycle life characteristics. In addition, electrode expansion may be suppressed during charge and discharge to help improve energy density.

In addition, within the ranges of the DD value, it means that the negative active material is oriented at a specific angle with respect to the current collector, and heat generated during charge and discharge of the battery using the negative electrode and heat generated by short-circuiting during penetration or collision may be vertically diffused and may be easily released outside. This may help suppress ignition caused by thermal runaway and may help suppress an internal temperature increase of the battery, and thus battery characteristics may be improved. When the DD value is less than 24, it indicates that the negative active material is arranged substantially horizontally in the current collector. In this case, generated heat is horizontally diffused, so that it is not easily released outside.

The DD value of the negative electrode within the ranges means that a negative active material included in a negative active material layer is oriented at a predetermined angle, and this DD value is maintained after charges and discharges. In an implementation, in order to orient a negative active material included in a negative active material layer at a predetermined angle, a magnetic field may be applied, while a negative active material composition is coated on a current collector.

In an implementation, the negative electrode satisfying the DD value may be manufactured by adjusting, e.g., an intensity of the magnetic field, a time of exposure to the magnetic field, and/or an electrode plate density during compression of the negative electrode.

Hereinafter, a method of manufacturing the negative electrode is described. As shown in FIG. 1 , the negative electrode may be manufactured by placing a current collector on and/or under a magnet, and then applying a negative active material composition including a negative active material on a current collector, exposing it to a magnetic field, and then drying and compressing.

In an implementation, the magnet may have an intensity of a magnetic field in a range of about 1,000 Gauss to about 10,000 Gauss. In an implementation, the negative active material composition may be coated on the current collector and maintained for about 3 seconds to about 9 seconds, e.g., may be exposed to the magnetic field for about 3 seconds to about 9 seconds. The electrode plate density during compression may be controlled so that the electrode density within the ranges may be obtained. The electrode plate density during compression may be about 1.4 g/cc to about 1.6 g/cc. As used herein, the electrode plate density refers to a density calculated by dividing a loading amount of a measured electrode plate by a thickness of the electrode plate during compression. As described above, the DD value may be adjusted within the ranges by controlling an intensity of the magnetic field, a time of exposure to the magnetic field, and an electrode plate density during compression of the negative electrode.

For example, when the coating process is performed as the current collector is moved, the magnetic field (magnetic flux) by the magnet may be formed vertically with the current collector, but the magnetic field according to a coating speed (a speed of moving the current collector) is formed at a predetermined angle as a vector function, and the negative active material included in the negative active material composition may stand, e.g., may be oriented at the predetermined angle on the surface of the current collector.

The negative electrode may have a peak intensity ratio at a (004) plane relative to a (002) plane, that is, I₍₀₀₄₎/I₍₀₀₂₎ of about 0.04 or greater and specifically, about 0.04 to about 0.07 when XRD is measured by using a CuKα ray. When the negative electrode has I₍₀₀₄₎/I₍₀₀₂₎ of about 0.04 or greater, DC internal resistance may not be increased, but rate capabilities and particularly, high-rate capability may be improved, and cycle-life characteristics may also be improved.

In an implementation, the negative electrode may have a peak intensity ratio at a (110) plane relative to a (004) plane, e.g., I₍₁₁₀₎/I₍₀₀₄₎ of greater than or equal to about 0.3 or greater than or equal to about 0.1, greater than or equal to about 0.2, about 0.3 to about 0.8, or about 0.3 to about 0.7. When the negative electrode has I₍₁₁₀₎/I₍₀₀₄₎ of greater than or equal to about 0.1, DC internal resistance may not be increased and rate capability, particularly high rate capability, and high-rate cycle-life characteristics may be improved.

In an implementation, the DD value is a peak value at a non-plane relative to a peak value at all the angles and thus not linked with I₍₁₁₀₎/I₍₀₀₄₎, and the I₍₁₁₀₎/I₍₀₀₄₎ of greater than or equal to about 0.1 does not mean the DD value of greater than or equal to about 24.

The negative electrode may have a peak intensity ratio, e.g., I_((101)H)/I₍₀₀₄₎ of (101)H plane relative to (004) plane of greater than or equal to about 0.4 or about 0.4 to about 3.0. When I_((101)H)/I₍₀₀₄₎ of the negative electrode is greater than or equal to about 0.4, DC internal resistance may not be increased and rate capability, particularly high rate capability, and high-rate cycle-life characteristics may be improved.

In an implementation, the DD value may be a value obtained by charging/discharging a rechargeable lithium battery including the negative electrode, disassembling the battery in a fully discharged state, and then measuring an XRD of the negative electrode. In an implementation, the charge and discharge may be performed once to twice at 0.1 C to 0.2 C.

In an implementation, the electrode density of the negative electrode may be about 1.0 g/cc to about 1.5 g/cc, e.g., about 1.0 g/cc to about 1.4 g/cc or about 1.0 g/cc to about 1.2 g/cc.

As used herein, the electrode density refers to a density calculated by dividing a loading amount of a measured electrode by a thickness of the electrode after charging and discharging once or twice at 0.1 C to 0.2 C under a condition that the electrode is sufficiently impregnated with the electrolyte solution.

When the electrode density of the negative electrode is within the ranges, high-rate charge and discharge characteristics, cycle-life characteristics, e.g., high-rate cycle-life characteristics, may be effectively improved.

In an implementation, the negative electrode may have a single surface loading level (L/L) of about 6 mg/cm² to about 65 mg/cm². Within the range, a desirable electrode density may be easily obtained.

The carbon negative active material may be artificial graphite or a mixture of artificial graphite and natural graphite. When the negative active material is a crystalline carbon material obtained by mixing natural graphite with artificial graphite or artificial graphite, the crystalline carbon material may have more developed crystalline characteristics than an amorphous carbon active material and thus may further improve orientation characteristics of the carbon material in an electrode plate about an external magnetic field. In an implementation, the artificial graphite or natural graphite may be amorphous, sheet-shaped, flake-shaped, spherically-shaped, fiber-shaped, or a combination thereof. In an implementation, the artificial graphite may be mixed with the natural graphite in a weight ratio of about 5:95 to about 95:5, e.g. about 70:30 to about 30:70.

In an implementation, the negative active material layer may further include, e.g., a Si negative active material, a Sn negative active material, or a lithium vanadium oxide negative active material. When the negative active material layer further includes these materials, e.g., the carbon negative active material as a first negative active material and at least one from the negative active material described above as a second negative active material, the first and second negative active materials may be mixed in a weight ratio of about 50:50 to about 99:1.

The Si negative active material may be Si, a Si—C composite, SiO_(x) (0<x≤2), and a Si-Q alloy (wherein Q is an element selected from an alkali metal, an alkaline-earth metal, a Group 13 element, a Group 14 element, a Group 15 element, a Group 16 element, a transition metal, a rare earth element, and a combination thereof but not Si), and the Sn negative active material is selected from Sn, SnO₂, a Sn—R alloy (wherein R is an element selected from an alkali metal, an alkaline-earth metal, a Group 13 element, a Group 14 element, a Group 15 element, a Group 16 element, a transition metal, a rare earth element, and a combination thereof but not Sn), and the like and also, a mixture of at least one thereof with SiO₂. The elements Q and R may be selected from Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Tl, Ge, P, As, Sb, Bi, S, Se, Te, Po, and a combination thereof.

According to another embodiment, the negative electrode may have an active region facing or overlying a positive electrode and an inactive region not facing or not overlying the positive electrode. For example, as shown in FIG. 2 , a region (A) of the negative electrode facing or overlying the positive electrode may be the active region, and another region (B) of the negative electrode not facing or not overlying the positive electrode may be the inactive region. The reason is that the inactive region where the negative electrode does not face or overlie the positive electrode is formed, since the negative electrode may be manufactured to be larger than the positive electrode to help improve battery safety. As the inactive region is generated, safety regarding a short circuit between the positive and negative electrodes due to lithium deposition on the surface of the negative electrode (which could occur during charging) may be improved, and the inactive region has relatively larger resistance of lithium ions than the positive electrode than the active region facing the positive electrode due to a long moving path of lithium ions transmitted from the positive electrode and is present as a non-buffering region. The DD value of the inactive region may be increased up to 24, and lithium may be easily diffused and capacity may be increased due to decrease of a non-buffering region.

In an implementation, the DD values of the active region and the inactive region may be about 24 or greater, e.g., about 24 to about 70. In an implementation, the DD value of only the inactive region may be about 24 or greater, e.g., about 24 to about 60. When the DD value of only the inactive region is about 24 or greater, the DD value of the active region may be a suitable value.

In the negative active material layer, an amount of the negative active material may be about 95 wt % to about 99 wt %, based on a total weight of the negative active material layer.

In an implementation, the negative active material layer may include a binder, and optionally a conductive material. In the negative active material layer, an amount of the binder may be about 1 wt % to about 5 wt %, based on a total amount of the negative active material layer. When the conductive material is included, about 90 wt % to about 98 wt % of the negative active material, about 1 wt % to about 5 wt % of the binder, and about 1 wt % to about 5 wt % of the conductive material may be used.

The binder may help improve binding properties of negative active material particles with one another and with a current collector. The binder may be a non-aqueous binder, an aqueous binder, or a combination thereof.

The non-aqueous binder may include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinylfluoride, an ethylene oxide-containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, or a combination thereof.

The aqueous binder may include a styrene-butadiene rubber, an acrylated styrene-butadiene rubber (SBR), an acrylonitrile-butadiene rubber, an acrylic rubber, butyl rubber, an ethylenepropylene copolymer, polyepichlorohydrine, polyphosphazene, polyacrylonitrile, polystyrene, an ethylenepropylenediene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyesterresin, an acrylic resin, a phenolic resin, an epoxy resin, polyvinyl alcohol, an acrylate resin, or a combination thereof.

When the aqueous binder is used as a negative electrode binder, a cellulose compound may be further included to provide viscosity as a thickener. The cellulose compound includes one or more of carboxylmethyl cellulose, hydroxypropylmethyl cellulose, methyl cellulose, or alkali metal salts thereof. The alkali metal may be Na, K, or Li. The thickener may be included in an amount of about 0.1 to about 3 parts by weight based on 100 parts by weight of the negative active material.

The conductive material may be included to provide electrode conductivity. A suitable electrically conductive material that does not cause a chemical change in a battery may be used as a conductive material. Examples of the conductive material may include a carbon material such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, a carbon fiber and the like; a metal material of a metal powder or a metal fiber including copper, nickel, aluminum, silver, and the like; a conductive polymer such as a polyphenylene derivative; or a mixture thereof.

The current collector may include a copper foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, a polymer substrate coated with a conductive metal, or a combination thereof.

A rechargeable lithium battery according to another embodiment includes the negative electrode, a positive electrode, and an electrolyte.

The rechargeable lithium battery may be a high power battery. For example, the rechargeable lithium battery may be usefully applied to an electronic device requiring high power such as a power tool, an electric vehicle, a vacuum cleaner, and the like. The reason is that the rechargeable lithium battery including the negative electrode according to an embodiment may easily release heat generated during the charge and discharge, e.g., when applied to a high-capacity battery and an electronic device for high power and thus may be suppressed from deterioration due to the heat and effectively used as a high power battery. In addition, the rechargeable lithium battery may easily release heat according to the charge and discharge and be effectively suppressed from a battery temperature increase and thus effectively improve cycle-life characteristics and particularly, cycle-life characteristics at a high rate.

This high power battery may be a cylindrical battery, a prismatic battery, or a pouch-type battery.

The positive electrode may include a positive current collector and a positive active material layer formed on the positive current collector. The positive active material may include lithiated intercalation compounds that reversibly intercalate and deintercalate lithium. For example, one or more composite oxides of a metal selected from cobalt, manganese, nickel, and a combination thereof, and lithium may be used. For example, the compounds represented by one of the following chemical formulae may be used. Li_(a)A_(1-b)X_(b)D₂ (0.90≤a≤1.8, 0≤b≤0.5); Li_(a)A_(1-b)X_(b)O_(2-c)D, (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05); Li_(a)E_(1-b)X_(b)O_(2-c)D_(c) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05); Li_(a)E_(2-b)X_(b)O_(4-c)D_(c) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05); Li_(a)Ni_(1-b-c)Co_(b)X_(c)D_(a) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.5, 0<α≤2); Li_(a)Ni_(1-c)Co_(b)X_(e)O_(2c-α)T_(α), (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, 0<α<2); Li_(a)N_(1-b-c)Co_(b)X_(c)O_(2-α)T₂ (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, 0<α<2); Li_(a)N_(1-b-c)Mn_(b)X_(c)D_(α) (0.90≤a≤1.8, ≤b≤0.5, 0≤c≤0.05, 0≤α≤2); Li_(a)N_(1-b-c)Mn_(b)X_(c)O_(2-α)T_(α) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, 0≤α≤2); Li_(a)Ni_(1-b-c) Mn_(b)X_(c)O_(2-α)T₂ (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, 0≤α≤2); Li_(a)Ni_(b)E_(c)G_(d)O₂ (0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5, 0.001≤d≤0.1); Li_(a)Ni_(b)Co_(c)Mn_(d)Ge_(c)O₂ (0.90≤a≤1.8, 0 b≤0.9, 0≤c≤0.5, 0≤d≤0.5, 0.001≤e≤0.1); Li_(a)NiG_(b)O₂ (0.90≤a≤1.8, 0.001≤b≤0.1); Li_(a)CoG_(b)O₂ (0.90≤a≤1.8, 0.001≤b≤0.1); Li_(a)Mn_(1-b)G_(b)O₂ (0.90≤a≤1.8, 0.001≤b≤0.1); Li_(a)Mn₂G_(b)O₄ (0.90≤a≤1.8, 0.001≤b≤0.1); Li_(a)Mn_(1-g)G_(g)PO₄ (0.90≤a≤1.8, 0≤g≤0.5); QO₂; QS₂; LiQS₂; V₂O₅; LiV₂O₅; LiZO₂; LiNiVO₄; Li_((3-f))J₂(PO₄)₃ (0≤f≤2); Li_((3-f))Fe₂(PO₄)₃(0≤f≤2); or Li_(a)FePO₄ (0.90≤a≤1.8).

In the above chemical formulae, A is selected from Ni, Co, Mn, and a combination thereof; X is selected from Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, a rare earth element, and a combination thereof; D is selected from O, F, S, P, and a combination thereof, E is selected from Co, Mn, and a combination thereof, T is selected from F, S, P, and a combination thereof; G is selected from Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, and a combination thereof; Q is selected from Ti, Mo, Mn, and a combination thereof; Z is selected from Cr, V, Fe, Sc, Y, and a combination thereof, and J is selected from V, Cr, Mn, Co, Ni, Cu, and a combination thereof.

The compounds may have a coating layer on the surface, or may be mixed with another compound having a coating layer. The coating layer may include at least one coating element compound selected from an oxide of a coating element, a hydroxide of a coating element, an oxyhydroxide of a coating element, an oxycarbonate of a coating element, and a hydroxyl carbonate of a coating element. The compound for the coating layer may be amorphous or crystalline. The coating element included in the coating layer may include Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or a mixture thereof. The coating layer may be disposed by a method having no adverse influence on properties of a positive active material by using these elements in the compound. For example, the method may include any coating method (e.g., spray coating, dipping, etc.).

In the positive electrode, a content of the positive active material may be about 90 wt % to about 98 wt % based on a total weight of the positive active material layer.

In an implementation, the positive active material layer may further include a binder and a conductive agent. In an implementation, the binder and the conductive agent may be included in an amount of about 1 wt % to about 5 wt %, respectively, based on the total amount of the positive active material layer.

The binder improves binding properties of positive active material particles with one another and with a current collector. Examples thereof may include polyvinyl alcohol, carboxylmethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinylchloride, carboxylated polyvinylchloride, polyvinylfluoride, an ethylene oxide-containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene butadiene rubber, acrylated styrene butadiene rubber, an epoxy resin, nylon, and the like.

The conductive material is included to provide electrode conductivity. A suitable electrically conductive agent that does not cause a chemical change may be used. Examples of the conductive material may include a carbon material such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, a carbon fiber and the like; a metal material of a metal powder or a metal fiber including copper, nickel, aluminum, silver, and the like; a conductive polymer such as a polyphenylene derivative; or a mixture thereof.

In an implementation, the current collector may use Al.

The electrolyte may include a non-aqueous organic solvent and a lithium salt.

The non-aqueous organic solvent serves as a medium for transmitting ions taking part in the electrochemical reaction of a battery.

The non-aqueous organic solvent may include a carbonate, ester, ether, ketone, alcohol, or aprotic solvent.

The carbonate solvent may include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and the like. The ester solvent may include methyl acetate, ethyl acetate, n-propyl acetate, dimethylacetate, methylpropionate, ethylpropionate, decanolide, mevalonolactone, caprolactone, and the like. The ether solvent may include dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, and the like. The ketone solvent includes cyclohexanone, and the like. The alcohol solvent may include ethyl alcohol, isopropyl alcohol, and the like. Examples of the aprotic solvent include nitriles such as R-CN (where R is a C2 to C20 linear, branched, or cyclic hydrocarbon, and where R may comprises a double bond, an aromatic ring, or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, sulfolanes, and the like.

The organic solvent may be used alone or in a mixture. When the organic solvent is used in a mixture, the mixture ratio may be controlled in accordance with a desirable battery performance.

The carbonate solvent may include a mixture with a cyclic carbonate and a chain carbonate. The cyclic carbonate and chain carbonate may be mixed together in a volume ratio of about 1:1 to about 1:9. When the mixture is used as an electrolyte, it may have enhanced performance.

The organic solvent may further include an aromatic hydrocarbon solvent as well as the carbonate solvent. The carbonate solvent and aromatic hydrocarbon solvent may be mixed together in a volume ratio of about 1:1 to about 30:1.

The aromatic hydrocarbon organic solvent may include an aromatic hydrocarbon compound represented by Chemical Formula 1.

In Chemical Formula 1, R₁ to R₆ are the same or different and may be selected from hydrogen, a halogen, a C1 to C10 alkyl group, a haloalkyl group, and a combination thereof.

Examples of the aromatic hydrocarbon organic solvent may include benzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene, 1,2,3-triiodobenzene, 1,2,4-triiodobenzene, toluene, fluorotoluene, 2,3-difluorotoluene, 2,4-difluorotoluene, 2,5-difluorotoluene, 2,3,4-trifluorotoluene, 2,3,5-trifluorotoluene, chlorotoluene, 2,3-dichlorotoluene, 2,4-dichlorotoluene, 2,5-dichlorotoluene, 2,3,4-trichlorotoluene, 2,3,5-trichlorotoluene, iodotoluene, 2,3-diiodotoluene, 2,4-diiodotoluene, 2,5-diiodotoluene, 2,3,4-triiodotoluene, 2,3,5-triiodotoluene, xylene, and a combination thereof.

The electrolyte may further include an additive of vinylene carbonate, an ethylene carbonate compound represented by Chemical Formula 2, or propanesultone to improve a cycle life.

In Chemical Formula 2, R₇ and R_(g) may each independently be hydrogen, a halogen, a cyano group (CN), a nitro group (NO₂), or a C1 to C5 fluoroalkyl group, provided that at least one of R₇ and R₈ is a halogen, a cyano group (CN), a nitro group (NO₂), or a C1 to C5 fluoroalkyl group, and R₇ and R₈ are not simultaneously hydrogen.

Examples of the ethylene carbonate compound may include difluoro ethylenecarbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, or fluoroethylene carbonate. The amount of the additive for improving cycle life may be flexibly used within an appropriate range.

The lithium salt dissolved in an organic solvent supplies a battery with lithium ions, basically operates the rechargeable lithium battery, and improves transportation of the lithium ions between positive and negative electrodes. Examples of the lithium salt may include LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiN(SO₂C₂F₅)₂, Li(CF₃SO₂)₂N, LiN(SO₃C₂F₅)₂, LiC₄F₉SO₃, LiClO₄, LiAlO₂, LiAlCl₄, LiN(C_(x)F_(2x+1)SO₂)(CyF_(2y+1)SO₂) (where x and y are natural numbers, for example integers of 1 to 20), LiCl, LiI, and LiB(C₂O₄)₂ (lithium bis(oxalato) borate; LiBOB). A concentration of the lithium salt may range from about 0.1 M to about 2.0 M. When the lithium salt is included at the above concentration range, an electrolyte may have excellent performance and lithium ion mobility due to optimal electrolyte conductivity and viscosity.

A separator may be included between the positive electrode and the negative electrode depending on types of the rechargeable lithium battery. Examples of a suitable separator material include polyethylene, polypropylene, polyvinylidene fluoride, and multi-layers thereof such as a polyethylene/polypropylene double-layered separator, a polyethylene/polypropylene/polyethylene triple-layered separator, and a polypropylene/polyethylene/polypropylene triple-layered separator.

FIG. 3 illustrates an exploded perspective view of a rechargeable lithium battery according to an embodiment. In an implementation, the rechargeable lithium battery may be a cylindrical battery, a prismatic battery, or a pouch battery.

Referring to FIG. 3 , a rechargeable lithium battery 1 according to an embodiment includes an electrode assembly including a positive electrode 2; a negative electrode 4; and a separator 3 disposed between the positive electrode 2 and the negative electrode 4; an electrolyte solution (not shown) impregnated in the electrode assembly; a battery case 5 housing the electrode assembly; and a sealing member 6 sealing the battery case 5.

The following Examples and Comparative Examples are provided in order to highlight characteristics of one or more embodiments, but it will be understood that the Examples and Comparative Examples are not to be construed as limiting the scope of the embodiments, nor are the Comparative Examples to be construed as being outside the scope of the embodiments. Further, it will be understood that the embodiments are not limited to the particular details described in the Examples and Comparative Examples.

Example 1

97.5 wt % of artificial graphite, 1.5 wt % of a styrene butadiene rubber, and 1 wt % of carboxylmethyl cellulose were mixed in a water solvent to prepare negative active material slurry.

After disposing a Cu foil on a magnet having an intensity of a magnetic field of 3,000 Gauss, the negative active material slurry was coated on the Cu foil, exposed to the magnetic field for 9 seconds, while the Cu foil was being moved, and then, dried and compressed to manufacture a negative electrode having an electrode plate density of 1.45 g/cc and a single surface loading level (L/L) of 6.2 mg/cm².

Positive active material slurry was prepared by mixing 96 wt % of a LiCoO₂ positive active material, 2 wt % of a carbon black conductive material, and 2 wt % of a polyvinylidene fluoride binder in a N-methylpyrrolidone solvent. The slurry was coated on an Al substrate and then, dried and compressed to manufacture a positive electrode.

The negative electrode, the positive electrode, and an electrolyte were used to manufacture a pouch-type rechargeable lithium battery cell having cell capacity of 550 mAh and current density of 2.63 mAh/cm². Herein, the electrolyte was prepared as a 1 M LiPF₆ solution in a mixed solvent of ethylene carbonate and diethyl carbonate (volume ratio of 50:50).

Example 2

A negative electrode and a rechargeable lithium battery cell using the same were manufactured according to the same method as Example 1 except that the electrode plate density was 1.4 g/cc.

Example 3

A negative electrode and a rechargeable lithium battery cell using the same were manufactured according to the same method as Example 1 except that the electrode plate density was 1.6 g/cc.

Example 4

A negative electrode and a pouch-type rechargeable lithium battery cell using the same were manufactured according to the same method as Example 1 except that the exposure time to the magnetic field was changed from 9 seconds to 5 seconds.

Comparative Example 1

A negative active material slurry was prepared by mixing 97.5 wt % of artificial graphite, 1.5 wt % of a styrene butadiene rubber, and 1 wt % of carboxylmethyl cellulose in a water solvent.

The negative active material slurry was coated on a Cu foil and then, dried and compressed to manufacture a negative electrode having an electrode plate density of 1.45 g/cc and a single surface loading level (L/L) of 6.2 mg/cm².

The negative electrode was used along with the positive electrode and the electrolyte used in Example 1 to manufacture a pouch-type rechargeable lithium battery cell.

Comparative Example 2

A negative electrode and a rechargeable lithium battery cell using the same were manufactured according to the same method as Example 1 except that the electrode plate density was 1.70 g/cc.

Comparative Example 3

A negative electrode and a rechargeable lithium battery cell using the same were manufactured according to the same method as Example 1 except that the electrode plate density was 1.79 g/cc.

Comparative Example 4

A negative electrode and a pouch-type rechargeable lithium battery cell using the same were manufactured according to the same method as Example 1 except that the exposure time to the magnetic field was changed from 9 seconds to 2 seconds.

Comparative Example 5

A negative electrode and a pouch-type rechargeable lithium battery cell using the same were manufactured according to the same method as Example 1 except that the electrode plate density was 1.70 g/cc and the exposure time to the magnetic field was changed from 9 seconds to 6 seconds.

Comparative Example 6

A negative electrode and a pouch-type rechargeable lithium battery cell using the same were manufactured according to the same method as Example 1 except that the electrode plate density was 1.79 g/cc and the exposure time to the magnetic field was changed from 9 seconds to 4 seconds.

Measurement of X-ray Diffraction Characteristics

The rechargeable lithium battery cells according to Examples 1 to 4 and Comparative Examples 1 to 4 were twice charged and discharged at 0.1 C and then completely discharged down to 2.75 V at 0.1 C. The completely-discharged battery cells were disassembled to obtain negative electrodes. As for these negative electrodes, an X′Pert (PANalytical B.V.) XRD equipment using a CuKα ray as a target ray was used, and a monochromator was removed in order to improve a peak intensity resolution. Herein, the measurement was performed under a condition of 20=10° to 80°, a scan rate (°/S)=0.06436, and a step size of 0.026°/step.

The measured XRD results are shown in FIG. 4 (Example 1) and FIG. 5 (Comparative Example 1). As shown in FIGS. 4 and 5 , the negative electrode of Comparative Example 1 showed a higher peak at 2θ=26.5±0.2° than the negative electrode of Example 1. In addition, as shown in FIGS. 4 and 5 , the negative electrodes of Example 1 and Comparative Example 1 showed peaks at 2θ=26.5±0.2° ((002) plane), 42.4±0.2° ((100) plane), 43.4±0.2° ((101)R plane), 44.6±0.2° ((101)H plane), 54.7±0.2° ((004) plane), and 77.5±0.2° ((110) plane).

As shown in FIG. 4 and FIG. 5 , the areas of the peaks shown at 2θ=26.5±0.2° ((002) plane), 42.4±0.2° ((100) plane), 43.4±0.2° ((101)R plane), 44.6±0.2° ((101)H plane), 54.7±0.2° ((004) plane), and 77.5±0.2° ((110) plane) were measured, and the results are shown in Table 1. In addition, the area sum of peaks shown at 2θ=42.4±0.2° ((100) plane), 43.4±0.2° ((101)R plane), 44.6±0.2° ((101)H plane), 77.5±0.2° ((110)plane) as I_(a), the area sum of peaks shown at 2θ=26.5±0.2° ((002) plane), 42.4±0.2° ((100) plane), 43.4±0.2° ((101)R plane), 44.6±0.2° ((101)H plane), 54.7±0.2° ((004) plane), and 77.5±0.2° ((110) plane) are shown in Table 1 as I_(total) and DD values ((I_(a)/I_(total))*100) were calculated therefrom and also shown in Table 1. Furthermore, I₍₀₀₄₎/I₍₀₀₂₎ and I_((101)H)/I₍₀₀₄₎ were calculated and are shown in Table 1. In particular, peaks corresponding to the (101)R plane of graphite and the (111) plane of a Cu current collector at 43.4±0.2° were overlapped.

TABLE 1 Peak plane Peak area 2θ (peak index) Comparative Example 1 Example 1 26.5 ± 0.2° (002) 213953.00 48241.35 42.4 ± 0.2° (100) 444.15 884.48 43.4 ± 0.2° (101)R 38786.60 37534.77 44.6 ± 0.2° (101)H 1143.22 3961.18 54.7 ± 0.2° (004) 6737.48 2135.52 77.5 ± 0.2° (110) 317.47 830.47 I_(total) 261381.90 93587.77 I_(a) 40691.41 43210.90 DD 15.60 46.20 I₍₀₀₄₎/I₍₀₀₂₎ 0.03 0.04 I₍₁₁₀₎/I₍₀₀₄₎ 0.047 0.389 I_((101)H)/I₍₀₀₄₎ 0.169 1.855

XRD of the rechargeable lithium battery cells according to Examples 2 to 4 and Comparative Examples 2 to 4 were measured according to the same method as those of Example 1 and Comparative Example 1, and used to calculate a DD value, I₍₀₀₄₎/I₍₀₀₂₎, I₍₁₁₀₎/I₍₀₀₄₎, and I_((101)H)/I₍₀₀₄₎, and the results are shown in Table 2. The DD value, I₍₀₀₄₎/I₍₀₀₂₎, I₍₁₁₀₎/I₍₀₀₄₎, and I_((101)H)/I₍₀₀₄₎ of the rechargeable lithium battery cells of Example 1 and Comparative Example 1 also are shown in Table 2 for comparison with the above results.

Measurement of Single Surface Loading Level, Thickness, and Electrode Density

The rechargeable lithium battery cells according to Examples 1 to 4 and Comparative Examples 1 to 4 were twice charged and discharged at 0.1 C and then completely discharged down to 2.75 V at 0.1 C. The completely-discharged battery cells were disassembled to obtain negative electrodes. The loading level (L/L) of the negative electrodes, single surface thicknesses of negative active material layers except current collectors, and electrode density were measured and shown in Table 2.

TABLE 2 Thickness Single surface (single surface, loading level except the Electrode DD I₍₀₀₄₎/ I₍₁₁₀₎/ I_((101)H)/ (L/L) substrate thickness) density value I₍₀₀₂₎ I₍₀₀₄₎ I₍₀₀₄₎ (mg/cm²) (μm) (g/cc) Example 1 46.20 0.04 0.389 1.855 6.2 47.7 1.3 Example 2 52 0.041 0.6 2.76 6.1 61 1.0 Example 3 40 0.058 0.387 1.99 6.2 41.3 1.5 Example 4 24.6 0.0459 0.144 0.425 6.4 49.2 1.3 Comparative 15.60 0.03 0.047 0.169 6.2 47.7 1.3 Example 1 Comparative 35 0.0478 0.201 0.09 6.1 39.4 1.55 Example 2 Comparative 30.8 0.044 0.235 0.957 6.3 39.4 1.6 Example 3 Comparative 20.8 0.051 0.08 0.21 6.5 50 1.3 Example 4 Comparative 30.0 0.035 0.054 0.049 6.1 39.4 1.55 Example 5 Comparative 23.0 0.025 0.122 0.085 6.3 39.4 1.6 Example 6

Referring to Table 2, the negative electrodes according to Examples 1 to 4 exhibited DD values of 24 or greater (24.6 to 52) and electrode densities of 1.0 g/cc to 1.5 g/cc. In addition, I₍₀₀₄₎/I₍₀₀₂₎ was greater than or equal to 0.04, I₍₁₁₀₎/I₍₀₀₄₎ was in a range of greater than or equal to 0.1, and I_((101H))/I₍₀₀₄₎ was in a range of greater than or equal 0.4.

In addition, Comparative Example 5 exhibited a DD value of 30 which is greater than or equal to 24 but I₍₁₁₀₎/I₍₀₀₄₎ of 0.054, which is less than 0.1, Comparative Example 6 exhibited a DD value of 23, which is less than 24 but I₍₁₁₀₎/I₍₀₀₄₎ of 0.122, which is greater than or equal to 0.1, and accordingly, the DD value and the I₍₁₁₀₎/I₍₀₀₄₎ were not interlocked each other. In addition, Comparative Examples 2 and 3 exhibited a DD of 35 and 30.8, respectively but I₍₁₁₀₎/I₍₀₀₄₎ of 0.201 and 0.235, respectively, and accordingly, the DD value was not proportional to the I₍₁₁₀₎/I₍₀₀₄₎.

Evaluation of Rate Capability

The rechargeable lithium battery cells according to Examples 1 to 4 and Comparative Examples 1 to 6 were respectively once charged and discharged at each C-rate of 0.2 C, 1 C, 2 C, 5 C, and 7 C. Discharge capacity ratios of discharge capacity at each C-rate relative to discharge capacity at 0.2 C according to the rechargeable lithium battery cells regarding Examples 1 to 4 and Comparative Examples 1 to 4 were calculated, and the results are shown in FIG. 6 .

Referring to FIG. 6 , the rechargeable lithium battery cells according to Examples 1 to 4 satisfying the ranges of a DD value and electrode density (e.g., having a DD value of greater than 24 (24.6 to 52) and an electrode density of 1.0 to 1.5 g/cc) exhibited excellent rate capability compared with the rechargeable lithium battery cells according to Comparative Examples 1 and 4 (which did not sufficiently satisfy a DD value and electrode density, e.g., a DD value of less than 24, electrode density of 1.0 to 1.5 g/cc) and the rechargeable lithium battery cells according to Comparative Examples 2 and 3 (a DD value of greater than or equal to 24 and electrode density of greater than 1.5 g/cc).

Evaluation of Cycle-Life Characteristics

The rechargeable lithium battery cells according to Examples 1 to 4 and Comparative Examples 1 to 6 were constant current/constant voltage charged under a 1.5 C, 4.2 V, and 100 mA cut-off condition, paused for 10 minutes, constant current discharged under a 8.75 C and 2.5 V cut-off condition, and paused again for 30 minutes, which is regarded as one charge and discharge cycle, and 400 charges and discharges were performed. A ratio of discharge capacity at each cycle relative to discharge capacity at the 1^(st) cycle was calculated to obtain a capacity retention depending on a charge and discharge cycle. The results of Examples 1 to 4 and Comparative Examples 1 to 4 are shown in FIG. 7 .

As shown in FIG. 7 , the rechargeable lithium battery cells according to Examples 1 to 4 satisfying the ranges of a DD value and electrode density (e.g., a DD value of 24 or greater (24.6 to 52) and electrode density of 1.0 to 1.5 g/cc) maintained a capacity retention of greater than or equal to 78% even at the 400^(th) charge and discharge cycle, but the rechargeable lithium battery cells according to Comparative Examples 1 and 4 (which did no not satisfy a DD value and/or electrode density, e.g., a DD value of less than 24 of electrode density of 1.0 to 1.5 g/cc) and the rechargeable lithium battery cells according to Comparative Examples 2 and 3 (a DD value of 24 or greater and electrode density of greater than 1.5 g/cc) exhibited a sharply decreased capacity retention of less than 70% at the 400^(th) charge and discharge cycle.

By way of summation and review, as for negative active materials, various carbon materials such as artificial graphite, natural graphite, hard carbon, and the like may be used. Recently, a non-carbon negative active material such as silicon or tin has been considered in order to obtain high capacity.

One or more embodiments may provide a negative electrode for a rechargeable lithium battery having improved electrochemical characteristics such as cycle-life characteristics and rate capability.

DESCRIPTION OF SYMBOLS

-   -   1: rechargeable lithium battery     -   2: positive electrode     -   3: separator     -   4: negative electrode     -   5: battery case     -   6: sealing member

Example embodiments have been disclosed herein, and although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. In some instances, as would be apparent to one of ordinary skill in the art as of the filing of the present application, features, characteristics, and/or elements described in connection with a particular embodiment may be used singly or in combination with features, characteristics, and/or elements described in connection with other embodiments unless otherwise specifically indicated. Accordingly, it will be understood by those of skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims. 

What is claimed is:
 1. A negative electrode for a rechargeable lithium battery, the negative electrode comprising: a current collector; and a negative active material layer on the current collector, the negative active material including a carbon negative active material; wherein: an electrode density of the negative electrode is about 1.0 g/cc to about 1.5 g/cc, and a DD (Degree of Divergence) value as defined by the following Equation 1 is about 24 or greater, DD(Degree of Divergence)=(I _(a) /I _(total))*100  [Equation 1] in Equation 1, I_(a) is a sum of peak intensities at non-planar angles measured by XRD using a CuKα ray, and I_(total) is a sum of peak intensities at all angles measured by XRD using a CuKα ray.
 2. The negative electrode for a rechargeable lithium battery as claimed in claim 1, wherein the DD value of the negative electrode is about 24 to about
 70. 3. The negative electrode for a rechargeable lithium battery as claimed in claim 1, wherein the DD value of the negative electrode is about 24 to about
 60. 4. The negative electrode for a rechargeable lithium battery as claimed in claim 1, wherein the electrode density of the negative electrode is about 1.0 g/cc to about 1.4 g/cc.
 5. The negative electrode for a rechargeable lithium battery as claimed in claim 1, wherein the electrode density of the negative electrode is about 1.0 g/cc to about 1.2 g/cc.
 6. The negative electrode for a rechargeable lithium battery as claimed in claim 1, wherein the negative electrode has a single surface loading level (L/L) of about 6 mg/cm² to about 65 mg/cm².
 7. The negative electrode for a rechargeable lithium battery as claimed in claim 1, wherein the negative active material layer has a thickness of a single surface except a substrate of about 40 μm to about 650 μm.
 8. The negative electrode for a rechargeable lithium battery as claimed in claim 7, wherein the negative active material layer has a thickness of a single surface except a substrate of about 40 μm to about 450 μm.
 9. The negative electrode for a rechargeable lithium battery as claimed in claim 7, wherein the negative active material layer has a thickness of a single surface except a substrate of about 40 μm to about 200 μm.
 10. The negative electrode for a rechargeable lithium battery as claimed in claim 1, wherein the peak intensities are peak integral area values.
 11. The negative electrode for a rechargeable lithium battery as claimed in claim 1, wherein the carbon negative active material is artificial graphite or a mixture of artificial graphite and natural graphite.
 12. The negative electrode for a rechargeable lithium battery as claimed in claim 1, wherein the negative active material layer further includes a Si negative active material, a Sn negative active material, a lithium vanadium oxide, or a combination thereof.
 13. The negative electrode for a rechargeable lithium battery as claimed in claim 1, wherein: the negative electrode has an active region facing a positive electrode and an inactive region not facing the positive electrode, and a DD value of the inactive region is about 24 or greater.
 14. A rechargeable lithium battery, comprising: the negative electrode as claimed in claim 1; a positive electrode; and an electrolyte.
 15. The rechargeable lithium battery as claimed in claim 14, wherein the rechargeable lithium battery is a high power battery. 